Using carbon isotopes to constrain C4 expansion from Bahía Blanca to Mendoza in response to changes in precipitation during the Late Miocene

Abstract

Between 8 to 3 million years ago, C4 plants abruptly expanded and replaced many C3 plant species globally. This expansion was one of the most important ecological transitions of the Cenozoic, though the cause of this phenomenon is poorly understood. Several studies have provided evidence for regional increased seasonal precipitation intensity as a primary influence. However, the relationship between the timing of C4 expansion in South America and the intensification of the South American Monsoon (SAM) is not yet well constrained. In this study, we use carbon isotopes in paleosol preserved organic matter to constrain the timing of C4 grass expansion throughout Argentina, and determine geochemical climate proxy data to reconstruct precipitation changes. Paleosol samples were obtained across a northwest trending transect of 12 sites from coastal Bahia Blanca (Buenos Aires province) through the La Pampa province to the Cacheuta Basin in the Mendoza province at the base of the Andes. The samples from Bahia Blanca to La Pampa comprise the Cerro Azul Formation which spans 10 to 5.7 Ma. The samples from the Mendoza region comprise the Mariño (19.2-12.03 Ma), La Pilona (11.1-9.8 Ma), Tobas Angostura (9.8 Ma), Rio de los Pozos (8.7-7.4 Ma), and Mogotes (<7.4 Ma) formations. From these formations, we analyzed the A horizons of three pedotypes (Protosols, Argillisols, and Calcisols) for carbon isotopes to constrain the timing of C4 presence in the landscape. In the Mendoza region, δ13C ranged from −25.32‰ to −22.95‰. In the Cerro Azul formation, δ13C ranged from −25.68‰ to −21.33‰, with the highest abundance of C4 vegetation from the northern sites. Geochemical proxies (e.g., CIA-K, Paleosol Weathering Index, and the Paleosol Paleoclimate Model) will be used to determine mean annual precipitation and mean annual temperature from the paleosol B horizons at each site. Based on preliminary carbon isotope results, we hypothesize that the difference between the Mendoza and Cerro Azul sites may be attributed to regional differences in precipitation throughout the late Miocene, with a more pronounced C4 abundance noted in the northernmost sections of the Cerro Azul Formation where SAM influence may have been strongest, and smaller, pulsed increases in C4 abundance over time in the Mendoza region where SAM changes were likely limited.